D and D is a household name. Traveller once was also. What happened? The devil was not interested in Traveller. As Traveller was never accused of having been associated with Satanism like d&d was in the late 1980's due to teen death during "live" role playing. Negative attention is still attention.
Traveller was never a household name. It may have been common in the gaming community, but certainly not a household name. D&D's notoriety (for good or ill) made it a household name. You'll note that the kids in Stranger Things are playing D&D.
Two things to keep in mind. One, D&D has had perhaps better marketing in general. This game also grew into a more simplified versions as it progressed into each generation of the game. Two, I'm not attempting to say anything negative about the marketing of either product. I am just stating what I understand of marketing concepts and what becomes a household name.
D&D was inherently popular, it became the "generic" term for RPGs at the time. Every game was compared to D&D, but D&D was like "Coke". Then there were the scandals and stories that simply raised its market awareness.
Let ask this, how many know the name MacDonald's? Now, who knows the name, Carl's jr.? or even Jack-in-the-Box? All three are burger franchise chains. MacDonald's is known worldwide. It even has a negative attention associated to it. I've never heard anything negative with Carl's jr. or Jack-in-the-box. I also have never seen one of those or have eaten there. Carl's jr. is from the South East US, and Jack--Box is a West Coast US. I live in the North East. I only know of them from small sources.
McDonalds premiered the idea of the franchise. They spent a lot of money in the 70's marketing to children as well, with Ronald McDonald and Hamburglers. Jack In The Box notoriety came from a food poisoning scare that almost killed them. They're now leaders and innovators in food safety, something they take _very_ seriously. After that, they pioneered the "Jack" character who is probably more loved than Ronald McDonald ever was. Carl's Jr is in fact from Southern California, they purchased Hardees in the late '90's, which had a midwest and (I guess) SE presence.
Even though they been around basically same time, D & D is available at every game store. What system is promoted/marketed better? ...need I say more.?
D&D is more accessible. Early on, it's combat, treasure, and more combat and treasure. There are no accountants in D&D. Dungeon crawls are easy. The game is heroic. We used to just run around a geomorphic map, and roll random monsters and make stuff up. No rhyme or reason, just wandering halls, killing monsters. Playing, in all sense of what the word "play" means.
D&D benefited from the enormous budget of WotC after it purchased it from MtG. The D&D books production wise are top quality. They always have been. Traveller hasn't done well with production after TNE.
So, It was just the point...TSR/Wizards of the Cost/Hasbro, were simply better at Marketing...When nearly no one has heard of you and you have been around for forty years, just as long as a competitor, whom everyone has heard of, it is a case of Marketing and promotion. One Marketing plan worked, one did not. just how it is.
It's not just marketing and promotion. There are endless Fantasy RPGs, dime a dozen, and many made a serious play for mindshare back in the day. But D&D prevailed, then got the monster war chest of WotC, gasoline to the flame.